The subject matter disclosed herein relates to a magnetic resonance image generating method, a method for correcting the positions of magnetic resonance signals, and a magnetic resonance imaging apparatus.
A magnetic resonance imaging apparatus is an apparatus for generating magnetic resonance signals using a nuclear magnetic resonance phenomenon and photographing or imaging a tomographic image of a subject.
Since time is required for imaging in the magnetic resonance imaging apparatus, attempts to shorten the imaging time have been made by various methods.
There are known, for example, a half echo method for acquiring or collecting data slightly greater than half of generated echo signals utilizing the symmetry of an echo waveform relative to an echo time interval and the symmetry of k-space data in a frequency direction and calculating the remaining portions using the conjugate symmetry, a parallel imaging method for executing a phase encode-thinned sequence using a phased array coil comprised of a plurality of RF coils different in sensitivity distribution and performing a development process for removing wrap-around artifacts by matrix operation, thereby shortening an imaging time interval, etc. (refer to, for example, Japanese Unexamined Patent Publication No. 2005-198715).
In the half echo method, homodyne processing is executed to make up for missing data. In order to execute the homodyne processing, it is carried out by allowing the center of each echo signal to pass through a high pass filter and a low pass filter. Since the high pass filter and the low pass filter are placed in the center of a k space, there is a need to displace the center of an echo signal shifted from the center of the k space to the center of the k space. Since the peak of each echo signal is normally taken as the center of the echo signal, the peak of the echo signal is displaced to the center of the k space.
Due to influences such as non-uniformity of a rotating magnetic field (B1), however, the peaks of a plurality of echo signals received by a phased array coil do not necessarily coincide. Assuming that a plurality of echo signals received from a plurality of RF coils constituting a phased array coil are echo signals E01, E02 and E03 as shown in FIGS. 29(a)-29(f), for example, the peaks of the respective echo signals are respectively shifted from the center O of a k space by frequencies different from frequencies FE01, FE02 and FE03. Therefore, there is a need to displace the echo signals with different displacements for the purpose of displacing the peaks of the received echo signals to the frequency axial center O in the k space.
In the parallel imaging method, a relationship of phase between sensitivity distributions of coils at a calibration scan and an actual scan gets out of order when all echo signals are not displaced with the same displacement in a k space, thus leading to the occurrence of artifacts. Therefore, when the half echo method and the parallel imaging method are utilized in combination, an RF coil for a channel set as the reference is selected, and a displacement for displacing the peak of an echo signal received by the selected RF coil to the frequency axial center O in the k space is applied to all echo signals, whereby centering processing is executed on all the echo signals. Assuming that the echo signal received by the RF coil for the channel set as the reference is an echo signal E01 as shown in FIGS. 30(a)-30(f), for example, the echo signals E01, E02 and E03 are displaced to execute centering processing with a frequency FE01 as a displacement for displacing the peak of the echo signal E01 to the frequency axial center O in the k space.
Now, since imaging by the parallel imaging method has extended from local regions such as the head to a wide range of portions or regions such as the abdomen, the plural RF coils that constitute the phased array coil have been brought to multichanneling. Therefore, the difference between the signal intensities at the echo signals received by the RF coils that constitute the phased array coil has been brought to the fore.
When an echo signal low in signal intensity is affected by noise or the like as shown in FIGS. 31(a) and 31(b), the maximum value of the signal intensity thereof becomes a noise portion, and both the maximum value thereof and its peak P might not coincide with each other. Thus, when an RF coil for receiving the echo signal low in signal intensity is selected as a reference coil where it is affected by noise or the like, the noise portion of the echo signal is judged to be its maximum value. Therefore, a displacement for executing centering processing on all echo signals is calculated based on this echo signal. When all the echo signals are displaced based on the displacement, the actual peak P of the echo signal is not displaced to a frequency axial center O in a k space over all the echo signals, and a portion shifted from the actual peak P is displaced to the frequency axial center O in the k space. Therefore, artifacts might occur in a generated image.